Planets on short-period orbits are expected to be unstable to tidal dissipation and finally spiral in toward the host star because they transfer the angular momentum of the orbital motion through tidal dissipation inside the star (e.g., Levrard et al. 2009; Essick & Weinberg 2016). The rate of this orbital decay can help determine the efficiency of the tide dissipation. The decaying orbital period is expected to be observed through transit timing. For some planets, the cumulative shift in transit times may be about 100 s after 10 years (Birkby et al. 2014). Tentative detection of the orbital decay were reported for the systems OGLE-TR-113 and WASP-43 (Adams et al. 2010; Blecic et al. 2014), but have not been confirmed by further observations (Hoyer et al. 2016; Jiang et al. 2016). The rate of orbital decay may be used to determine the tidal quality parameter for the host star Q' that remains poorly constrained to date. Orbits of short-period planets can also precess as a result of planet-star interactions (e.g., Ragozzine & Wolf 2009). The planet shape departs from spherical symmetry, and the two bodies in the system - the planet and the host star - raise mutual tides. The non-spherical mass component of the gravitational field results in precession of the orbit. This apsidal rotation might be observed through precise timing of transits for non-zero orbital eccentricities. The total apsidal precession is the sum of components such as tidal bulges, rotation bulges, and relativistic effects. The precession rate may be used to determine the second-order Love number, which is related to the planet's internal density profile. Calculations show that both phenomena - orbital decay and apsidal precession - might be detected for the very-hot and bloated exoplanets using the method of precise transit-timing over a time span of about 10 yr.

Using a typical value of stellar Q'=1e6 for host stars (assuming they are Sun-like stars), we found that exoplanets listed below could spiral inward within a few millions years. For all these systems, the calculated cumulative shift in transit times is greater than 1 minute after 10 years, so detectable from the ground. Those systems were selected from the list of confirmed transiting exoplanets and are the best candidate for studying star-planet interactions. We propose to obtain new transit timing data to attempt to detect any deviations from linear ephemerides and put lower constraints on values of stellar Q'.

WASP-12 b: We have detected the apparent shortening of the orbital period that could be caused by orbital decay or apsidal precession (Maciejewski et al. 2016). New observations are needed to follow the TTV signal.

Remaining targets: Lower constraints on the values of stellar Q' will be determined using observations acquired by the end of 2017. Those initial results will be published in an introduction paper in early 2018.

PI: Gracjan Maciejewski gmac@umk.pl